The art of "candling" is a very old one. The term originally referred to the practice of holding an egg in front of a candle to inspect it for freshness, but the term has since been adopted to more highly technical applications in which objects having at least some degree of translucency are inspected on the basis of light that is directed through the object to the human eye or to some appropriate form of optical detector.
The application of the candling technique to the inspection of fish fillets is known. See for example H. E. Power "The Effect of Various Lighting Conditions on the Efficiency of Candling Cod Fillets for Detection of Parasites" J.F.R.B. Canada 15(4) 537-542 (1958); and G. H. Valdimarsson et al "Detection of Parasites in Fish Muscle by Candling Technique." J. Assoc. Off.ANAL.CHEM. Vol. 68 No. 3 (1985).
Marine fishes, including commercial species such as cod and herring, are susceptible to infection by a number of parasites. Although a large number of parasites are removed by washing and eviseration, some worms can remain embedded in the filleted portion of a fish. Unless detected and removed, these worms will remain in the flesh and eventually be passed on to the consumer. While the parasites will normally be killed either by cooking or by freezing, they may remain a problem if the fish fillets are eaten raw or are not properly cooked or frozen. In any event, even assuming that the parasites are killed, their presence in the flesh produces an aesthetically unacceptable product. In many countries, inspectors will reject fish products that contains parasites, even though these may have already been killed by freezing.
At present, the commercial detection and removal of fish parasites and other aesthetically unacceptable anomalies such as bone, or blood from bruises, is carried out by a relatively labor intensive method of candling. This involves human inspection of fish fillets as they are passed along a conveyor with a bright light directed to shine through the translucent flesh of the fillet. The method is relatively unreliable, especially if the anomaly is deeply embedded, because of human fatigue and the insufficient ability of the human eye to detect small anomalies in fish fillets that are relatively thick and/or relatively opaque. Because the parasites are less translucent than the flesh of the fish, the parasites can theoretically be detected. However, the attenuation and scattering of the light as it travels through the flesh make the identification of anomalies quite difficult, and it has been determined that the present day technique of candling of cod fillets only detects about 60% of parasites.
Since the early candling techniques were developed, many modifications have been made to the format of the candling table, but it still basically involves a light source beneath a working surface. Studies conducted to define the optimum wavelength of light for the detection of parasites have shown that unfiltered, white light is usually the most effective light source. Recent experiments by K. C. Watts et al (1980) "Search for an Optical Window for Cod Fillets" an unpublished report for Fisheries and Oceans, Canada, have investigated the use of collimated light in the detection of parasites in fish flesh. However, these experiments claimed to find that collimated light was less effective than conventional light sources, and the current recommendation by the Department of Fishery and Oceans, Canada, issued in 1983,is that the most effective candling table is one constructed with a five millimeter thick acrylic sheet with 45% translucency and a light source giving 1500 lux as measured 30 cm above the surface of the table.
A more recent study from Iceland recommends that the light source should be cool white, with a color temperature of 9200.degree. K. Valdimarsson et al in 1985 (cited above) used two 20 watt fluorescent tubes with a conventional candling table. This study suggested that the brightness above the light source should be three times greater than that of the outer field and that the brightness of the outer limit of the visual field should not be more than one tenth the inner field. The overhead light should be at least 500 lux.
Despite all this research and the few improvements that have been made to the candling procedure in recent years, the method remains basically the same and is still very inadequate for a number of reasons. Firstly, the procedure is not convenient for automation and is thus extremely costly in manpower, resulting in increased costs for fish processing and reduced plant productivity. Secondly, even candling with intense light is not totally reliable, especially in relation to relatively thick fillets, e.g. 20 to 25 mm, and also in relation to deeply embedded parasites or other anomalies.
Various workers in this field have suggested the use of other methods, such as ultrasonics, ultraviolet light, X-ray radiation, and acoustic techniques, but none of these methods has proved sufficiently satisfactory to be commercially adopted to date. Optical methods including scanned laser beams have been proposed.